The prior art includes closed-loop adaptive optical systems which use conventional adaptive optical approaches such as deformable mirrors, wavefront error sensors (WESs), drive electronics and processors with N servo-loops, where N equals the number of resolvable pixels to be controlled. The number of resolvable pixels to be controlled may be on the order of a few hundred to over ten thousand.
Another approach to closed-loop systems includes an optical scheme to replace the N hard-wired servo-loops, and exploits a spatial light modulator (SLM) in an all-optical closed-loop configuration (U.S. Pat. No. 5,046,824; D. M. Pepper). In this system, a local reference beam is required that is coherent with respect to an incoming aberrated beam. The local reference beam coherently combines with the input beam to form a spatial interference pattern that is applied to and thereby activates the SLM. The generation of the local reference beam is rather challenging and it thus complicates the system architecture. For example, the local reference beam may be generated by spatially filtering a part of the incoming beam, phase shifting this spatially filtered beam (for quadrature interference), and then recombining it interferometrically with the incident beam to form the necessary fringe pattern.
Another approach involves sampling a diffraction-limited sub-aperture of the incoming beam (possibly amplifying a single speckle), phase shifting it, and then interfering it with the remainder of the incoming aberrated beam.
Yet another approach is to phase-lock a local oscillator to the incoming distorted beam.
Yet another approach employs a radial shearing method, whereby a sub-aperture of large wavefront radius is used as the local reference. However, this approach is very inefficient in terms of processing the incoming photons and, in addition, limits the spatial frequency compensation capability of the adaptive optical system.
The above examples all suffer from a relatively low photon efficiency. Moreover, since a common path is not used for the interfering beams, the system is sensitive to vibrations, and further, a long coherence length source is needed for the aberrated beam with a coherence length greater than any path difference in the system.
An example of an adaptive optical closed-loop system of the prior art is shown in FIG. 1. This prior art system includes a spatial light modulator (SLM) 1, which functions as an integrated wavefront error sensor and spatial phase modulator in a monolithic package. The SLM 1 may be a liquid crystal light valve, a MEMS-based device, etc. The SLM 1 is a two-port device, whose basic function is to transfer an input intensity pattern at an input port 2 (in the form of an image, an interference pattern, etc.) into an output optical phase map. The input port 2 of SLM 1 typically consists of a photoconductive surface 2 with spatially resolvable (e.g., pixelated) channels, whereas the output port 3 consists of an optically addressable array of optical phase shifters (e.g., a liquid crystal thin film, an array of MEMS membranes, an array of electro-optic phase shifters, etc.). The SLM 1 may be configured in a reflection or transmission architecture—a reflection architecture is shown in FIG. 1 and the other figures. Typically, a high resolution imaging system (not shown) is associated with the SLM 1, which imaging system is used to address the two ports of the SLM 1. The imaging system may include a monolithic micro-lenslet array as a means by which to address the output port 3 (for minimal phase curvature and distortion), since, typically, piston-only corrections are required. Then, a multilens systems is preferably used to provide a one-to-one imaging of the output port 3 of the SLM 1 to its input port 2 so that all resolvable pixels of the wavefront beam are mapped from the front to the back side of the SLM 1. Typically, a non-inverting, unity magnification, flat-field three-element telecentric imaging system is employed to provide for this system need. These imaging system details are known to those skilled in the art and thus for the sake of brevity, are not discussed in further detail here.
The reader is directed to the following references for additional information regarding SLM's and this area of technology:                (1) “Spatial Light Modulator Technology—Materials, devices and Applications”, edited by Uzi Efron, Marcel Dekker, Inc. publisher, pp 619–643, the disclosure of which is hereby incorporated wherein by reference;        (2) “Single-pixel demonstration of innovative adaptive optics by use of a charge-transfer membrane light modulator”, by P. V. Mitchell et al., Optic Letters, vol. 18, no. 20, Oct. 15, 1993, pp 1748–1750, the disclosure of which is hereby incorporated wherein by reference; and        (3) Characteristics of innovative adaptive-optics that use membrane-based spatial light modulators”, C. J. Gaeta et al., J. Opt. Soc. Am. A, Vol. 11, No. 2, February 1994, pp 880–894, the disclosure of which is hereby incorporated wherein by reference.        
The object of the prior art system of FIG. 1 is to scrub wavefront distortion which has occurred to a beam 15 due to it having passed through a region 14 which imposed the wavefront distortion on beam 15, the distorted beam being identified by numeral 4 in the figures as it exits region 14. The system of FIG. 1 receives an input field, such as the wavefront-distorted (aberrated) beam 4, and generates, in real-time, a “scrubbed” output beam 5, which is relatively free of aberrations compared to the distorted input beam 4 (the amount by which the beam is scrubbed will depend on the effective gain and dynamic range of SLM 1). The distorted input beam 4 is referred to as the external reference, since it references the external distortions 14 that need to be corrected. Examples of path aberrations/distortions 14 include the effects of propagation through a turbulent atmosphere, a turbid liquid, a multimode optical fiber, an optical amplifier, etc. As a result of the adaptive optical processing by the system, the aberrated input beam 4 is stripped of most of its phase distortions, and emerges as a relatively clean plane wave 5 with minimal loss of photons. The operation of this system is referred to as “wavefront scrubbing.”
In addition to wavefront scrubbing, the system can also be used to generate a phase-conjugate replica 11 (a wavefront-reversed and aberration-reversed beam) of a readout beam 6. In this case, the same architecture may be used, but, in addition, a plane wave readout beam 6 is directed into the reverse direction of the scrubbed output beam 5, as shown in FIG. 1. After reflection by the SLM 1, the readout beam 6 emerges as a phase-conjugate replica 11 of the input beam 4 and, therefore, “undoes” the initial distortions imposed on the input beam 4 by region 14 when transiting through the same (but in a reciprocal direction) aberrated path 14, and it emerges from region 14 as an aberration-reduced output beam 12.
In general, the adaptive optical system senses the wavefront distortion of the input beam 4 by sampling a portion of the external reference beam 8, using, for example, a beam splitter 7 preferably just after the SLM 1. This sampled external reference 8, which has some residual aberrations from region 14 since the scrubbing is not 100% effective, is then directed to the backside of the SLM (to the photoconductive input port 2), where it interferometrically combines with a coherent non-aberrated beam referred to as a local reference beam 9. The resultant interference pattern is an intensity mapping of the phase distortion of the external reference 8 relative to the local reference 9. Note that the local reference beam 9 is typically a plane wave. In general, upon convergence, the servo aspect of the adaptive optical system of FIG. 1 results in a corrected output beam 5 that possesses the same phasefront as that of the local reference beam 9 (limited by the open-loop gain and dynamic range of the servo system and particularly of SLM 1). In order for this system to properly function, the local reference beam 9 is phase-shifted relative to the external reference beam 8 by preferably 90 degrees so that this pair of reference beams 8, 9 are preferably in quadrature and optimal convergence of the servo loop is achieved. Phase shifter 10 imposes a 90 degree phase shift on the local reference beam 9. Note that this phase shift is the only operation required by an outside processor to enable the system to function. Thus, a single servo-control will, in essence, provide sufficient information for millions (for example) of equivalent adaptive optical piston actuators (the output pixels of the SLM) to properly set the phasefront across the device for compensation of the wavefront distortions of the input beam 4. This system has been demonstrated to be capable of providing wavefront compensation of distorted laser beams. Note that, in the case of laser beams, a separate coherent source is used for the local reference beam 9, the coherence length of which is sufficient so as to enable the pair of reference beams 8, 9 to form an interference pattern of high fringe contrast on the photoconductive port 2 of SLM 1.
The local reference beam 9 may, in principle, be generated by beam splitting part of the external reference beam 8 and spatially filtering it using a conventional pinhole 20 with an amplitude stop of fixed diameter, as shown in FIG. 2. This approach is referred to as “self-referencing”, since the local reference beam 9 is derived from the external reference beam 8 itself. In this case, a spatial filter 20 with an amplitude-stop fixed diameter pinhole is used, and a downstream optical phase shifter 10 is used for quadrature operation of the interferometer (basically, a Mach-Zehnder interferometer 16 with one beam being a plane wave and the other beam possessing many spatial modes—Mach-Zehnder interferometers 16 typically have two legs which cause an interference pattern, with one of the legs having a phase shifter 10 therein). The resultant output interference fringe pattern is then imaged onto the input port 2 of SLM 1.
The performance of such prior art systems generally suffer from the following limitations:                (1) A separate path is required for the generation of the local reference, which can lead to vibration-induced or thermally induced degradation of the system (the interferometric legs of the prior art systems must be maintained with a precision of a fraction of a wavelength (approximately λ/10) in path-length-differential to assure quadrature operation);        (2) Since photons are lost in the spatial filtering operation of FIG. 2, that system is not photon efficient and suffers from significant losses;        (3) The added path length dictates the need for a laser or an optical source whose coherence length exceeds the path-length differences in the Mach-Zehnder interferometer (path length differences between the two interfering beams 8, 9); and        (4) The fact that the pinhole embodiment (FIG. 2) has a fixed diameter can lead to a degradation in performance. (It is to be noted that in accordance with one aspect of the present invention, the system may employ an amplitude pinhole with a variable diameter. In this case, the system has been shown, in simulations, to improve the convergence performance and dynamics (response time and Strehl ratio) of the closed-loop system.)        
The prior art further teaches that a coherent local reference 9 may be generated by expanding (i.e., magnifying) part of the external reference beam 8 so that a fraction of the magnified wavefront is nearly planar (a portion of a spherical wave is nearly planar when the radius of the spherical wave becomes large as a result of magnifying the beam). This approach is discussed in the Jun. 1, 2000 issue of Optics Letters (Vol 25, No. 11).
However, when using this approach, the performance of the system is compromised for reasons which include the following:                (1) Only a small fraction of the photons is utilized, resulting in a loss of performance;        (2) Low order spatial frequencies are not processed, thereby limiting the spatial bandwidth of the system and resulting in a non-planar converged wavefront; and        (3) Controlling the phase of the beams to realize quadrature of the external and local reference beams is not addressed in this approach. This limits the performance of the system as well as its robustness with respect to vibrations and other noise sources, as well as the need for a long coherence-length source, since the feedback-loop interferometer is not a common-path interferometer (as is utilized in the embodiments disclosed herein).        